In the following description, the term "engine" is herein used to mean an internal combustion engine, an electric motor and a hybrid power unit that includes an internal combustion engine and an electric motor.
In an automotive drive train including an engine and a CVT, varying a CVT ratio, i.e., a ratio=(input speed)/(output speed), brings about the following phenomena.
(1) At acceleration, varying a CVT ratio in a shift-down direction causes a drop in drive torque for acceleration during a period of shifting owing to an increase in equivalent inertia in drive train, failing to meet power demand for acceleration. PA1 (2) During a shift from acceleration mode to an ordinary drive mode, varying CVT ratio in a shift-up direction causes an increase in drive torque for acceleration during a period of shifting owing to a reduction in equivalent inertia in drive train, providing an unexpected acceleration feel to the vehicle driver. PA1 J1 is moment of inertia of the CVT input shaft due to masses of engine and intermediate components between the engine and the CVT input shaft, PA1 Gf is a final drive reduction ratio, PA1 .omega..sub.w is angular speed of a wheel driven by the final drive, and PA1 G is a ratio that is a speed ratio between the CVT input and output shafts. PA1 generating a target ratio, PA1 generating an inertia torque from the target ratio, PA1 generating a correction required to get engine torque change against the inertia torque, and PA1 generating the first control signal in response to the correction.
U.S. Pat. No. 5,790,968 (JP-A 7-239002) discloses a CVT ratio rate control system that controls a CVT ratio rate (=speed at which CVT ratio changes) based on consideration of change in inertia torque, that is, apparent torque derived due to changes in equivalent inertia. This known CVT control system adjusts CVT ratio rate in response to an output speed of CVT in such a manner as to restrain inertia torque from exceeding a predetermined value.
JP-A 8-177541 discloses an engine torque control system for an engine-CVT drive train. According to this known engine control system, upon occurrence of a downshift command for acceleration, a ratio rate due to the downshift is anticipated and a drop, in amount, of engine torque is anticipated based on the anticipated ratio rate. During a period between the downshift command and the subsequent initiation of ratio change, the engine ignition timing is retarded in accordance with the anticipated drop of engine torque for a reduction of the engine torque. After initiation of the ratio change in the downshift direction, the amount of the ignition timing retard is gradually reduced in accordance with the actual CVT ratio rate. This control is intended to provide smooth increase in engine torque upon acceleration.
The known CVT ratio rate control system may not sufficiently meet driver's acceleration demand because drop in CVT ratio rate to suppress inertia torque causes slow acceleration during CVT ratio change in downshift direction.
Thus, it would be desired to make compensation for reduction in inertia torque with CVT ratio rate maintained to meet driver's acceleration demand.
Inertia torque .DELTA.Te_inertia, which is derived from changes in equivalent inertia in engine caused by CVT ratio changes, can be expressed as, EQU .DELTA.Te_inertia=J1.multidot..omega..sub.w.multidot.Gf.multidot.(dG/dt) Eq. 1
where:
Variations of the speeds of input and output shafts of CVT are measured for calculation of CVT ratio. These measured values of the speeds may involve measurement errors and noises that are not predictable and difficult to remove by the present technology. Deviations of calculated values of the CVT ratio from variations of the CVT ratio are subject to such measurement errors and noises. Unless appropriately processed, the calculated values of the CVT ratio cannot be regarded as G in the equation (1). If these values are substituted for G, calculated values of dG/dt are greatly deviated from variations of dG/dt, causing great deviations of calculated values of T.sub.I, from variations of T.sub.I. Thus, using the calculated values of T.sub.I in altering engine torque will fail to meet driver's acceleration demand during downshift for acceleration.
FIGS. 8 to 13 show simulation results. In FIG. 8, the reference character A shows plotting of simulated measures resulting from superimposing random noise with the probability density of the normal distribution, where variance is 0.01, upon real variations of CVT ratio from 1 to 2 against time over period of 2 seconds. In FIG. 9(A), a curve B connects values resulting from repeating subtraction of a previously sampled old value of the real variations of CVT ratio from a currently sampled new value thereof over 0 to 2 seconds. In FIG. 9(B), the reference character C shows plotting of values resulting from repeating subtraction of a previously sampled old value of the measures in FIG. 8 from a currently sampled new value thereof over 0 to 2 seconds. The values shown in FIG. 9(A) may be regarded as dG/dt, where the real variations are substituted for G in equation (1). The values shown in FIG. 9(B) may be regarded as dG/dt, where the simulated measures shown in FIG. 8 are substituted for G in equation (1). Comparing FIG. 9(B) with FIG. 9(A) reveals that the values of FIG. 9(a) deviates from the corresponding values of FIG. 9(B) too much to represent the real variations of dG/dt. This means that values resulting from calculating equation (1) after substitution of the values of FIG. 9(B) for dG/dt will fail to represent real variations of the inertia torque .DELTA.Te_inertia.
Filters are often used to process measures for removal or at least reduction of noise component.
In FIG. 10, a curve as indicated by the reference character a shows real variations of the CVT ratio from 1 to 2 over a period of 2 seconds, while a curve indicated by the reference character b shows values resulting from filtering the real variations. Comparing the curve b with the curve a clearly shows that the filtering has caused a phase shift.
The measures of the CVT ratio as shown at A in FIG. 8 are subjected to the filtering process to give filtered measures. In FIG. 11, the reference character Cl shows plotting of values resulting from repeating subtraction of a previously sampled old value of the filtered measures from a currently sampled new value thereof over 2 seconds. The curve B in FIG. 9(A) is drawn in FIG. 11 for comparison with the plotting C1. Comparing the plotting C1 in FIG. 11 with the plotting C in FIG. 9(B) reveals that the noise component has been reduced. However, the values on the plotting C1 shown in FIG. 11 still suffer from undesired variations, so that they cannot be used as substitution for dG/dt in the equation (1).
FIGS. 12 and 13 illustrate test results from signal processing with another filter, which has a different characteristic.
In FIG. 12, a curve a shows real variations of the CVT ratio from 1 to 2 over a period of 2 seconds, while a curve indicated by the reference character b1 shows values resulting from filtering the real variations. Comparing the curve b1 with the curve b shown in FIG. 10 clearly shows that this filtering has caused a greater phase shift.
The measures of the CVT ratio as shown at A in FIG. 8 are subjected to the filtering by this another filter to give filtered measures. In FIG. 13, the reference character C2 shows plotting of values resulting from repeating subtraction of a previously sampled old value of the filtered measures from a currently sampled new value thereof over 2 seconds. The curve B of FIG. 9(A) is drawn in FIG. 13 for comparison with the plotting C2. Comparing the plotting C2 in FIG. 13 with the plotting C1 in FIG. 11 reveals that the noise component has been reduced. However, the deviation from the curve B has increased, so that the values on the plotting C2 cannot be used as substitution for dG/dt in the equation (1).
There is a need to meet driver's demand or expectation for vehicle performance during variations of CVT ratio. An object of the present invention therefore is to improve a control system for a vehicle drive train including an engine and a CVT such that the above-mentioned need is fulfilled.